The moving picture experts group (MPEG) video coding standard has been proposed for a variety of applications for video transmission and storage. Several applications such as Video On Demand and Trick-Play on Track Digital VTRs, for example, are more easily facilitated with a compressed signal having a lesser bitrate than that provided in certain of the MPEG profiles. The different applications have slightly different signal requirements, however a similar scaling apparatus may serve to reduce an original bitrate to a rate conducive to a respective application.
An MPEG coding standard has now been developed for a variety of applications which include terrestrial high definition television (HDTV), teleconferencing, satellite communication, direct broadcasting systems (DBS) and multimedia workstations. An MPEG-2 compressed bit stream may represent a compressed HDTV bit stream of relatively high data rate. If this signal is to be utilized on relatively narrow band channels it is necessary to reduce or scale its data down to a lower bit rate.
Consider a Video On-Demand system wherein a video file-server includes a storage device containing a library of MPEG encoded bit streams. The bit streams stored in the library are originally coded at a high quality (e.g. studio quality). A number of clients may request retrieval of any of these video programs at one particular time. The number of users and the quality of video delivered to the users is constrained by the outgoing channel capacity. This outgoing channel, which may be a cable bus or an ATM trunk for example, must be shared among the users who are granted service. Different users may require different levels of video quality, and the quality of a respective program will be based on the fraction of the total channel capacity allocated each user.
To simultaneously accommodate a plurality of users, the video file server may scale the stored bit streams to a reduced bit rate before they are delivered over the channel to the respective users. The quality of the resulting scaled bit stream should not be significantly degraded compared to the quality of a hypothetical bit stream obtained by coding the original source material at the reduced rate. Complexity and cost is not a critical factor because only the file server has to be equipped with the scaling hardware, not respective users.
In Trick-play on Track Digital VTR systems, the video is scaled to create a side track on video tape recorders. This side track contains very coarse quality video sufficient to facilitate trick-modes on the VTR (e.g. fast forward and reverse scan at different speeds). Complexity and cost of scaling hardware included in these devices is of significant concern, because the VTR is a mass consumer item subject to mass production.
Another application of scaling is Extended-Play Recording on Digital VTRs. In this application, video is broadcast to users' homes at a certain broadcast quality (.about.6 Mbps for standard definition video and .about.24 Mbps for high definition video). With a scaling feature in their video tape recorders, users may record the video at a reduced rate, akin to extended play, EP, mode on today's VHS recorders, thereby recording a greater quantity of video program material onto a tape at lower quality.
In scaling, the higher quality of the information in the original signal should be exploited as much as possible, and the resulting image quality of the new signal with a lower bit rate should be as high as possible, or as close as possible to one created by coding the original source video at the reduced rate. It is assumed that for a given data rate the original source is encoded in an optimal way.